The rapidly increasing complexity of vacuum processing and the prohibitive cost of clean room facilities, particularly in the semiconductor industry, has produced an unmet and growing need for miniature ionization gauges with good accuracy and stability.
In U.S. Pat. No. 5,128,617; 5,250,906; 5,296,817; and 5,422,573 (all of which patents are assigned to the assignee of the present application and hereby incorporated herein by reference) and in J. Vac. Sci. Technol. A12, 568 (1994); J. Vac. Sci. Technol. A12, 574 (1994); and J. Vac. Sci. Technol. A12, 580 (1994), new gauges are described which improve the accuracy and long term stability of ionization gauges by at least a factor of ten compared to prior art ionization gauges. These new gauges, trademarked STABIL-ION.RTM. Gauges by the assignee of the present application and hereafter referred to as STABIL-ION gauges, are well suited for their intended purpose and are of comparable size to the relatively large prior art, glass enclosed Bayard-Alpert (BA) type ionization gauges they replace.
Miniature glass enclosed BA ionization gauges are known but suffer from all of the problems with prior art ionization gauges. These problems are described in full detail in above-referenced U.S. Pat. Nos. 5,128,617; 5,250,906; 5,296,817; and 5,422,573. In addition to these problems, these miniature glass enclosed BA gauges suffer from very low sensitivity and relatively high lower pressure limits.
Miniature metal enclosed BA gauges are also known which eliminate the well-known problems with glass enclosures. However, inside the metal enclosure the electrode geometry is essentially the same as prior art BA gauges. These all metal prior art miniature gauges also suffer from very low sensitivity and relatively high lower pressure limits.
When Bayard-Alpert ion gauge electrode geometry is decreased in size much below the standard dimensions originally used by Bayard and Alpert, gauge sensitivity is decreased markedly, accuracy and long term stability are significantly reduced and the so-called X-ray effect increases. When STABIL-ION gauge geometries are miniaturized, there is some deterioration of performance for the following reasons. To obtain relatively high sensitivity the electron path lengths must be long. To obtain long electron path lengths with small grid diameter requires high grid transparency. High grid transparency requires small diameter grid wires relative to the grid wire spacing. The grid wire spacing must be maintained small compared to the cathode to grid spacing to prevent space charge saturation of electron emission at the cathode. The cathode to grid spacing must be small to achieve small overall gauge size. Thus, it can be understood that the grid wires must be of small diameter, say, 0.002 in. in a miniaturized STABIL-ION.RTM. design, and thus are not self-supporting as in conventional glass BA gauges. Thus, axially extending grid supports must be used to support the small diameter grid wires to assure stable geometry. It is these relatively large diameter grid supports that cause difficulties in miniaturizing the STABIL-ION design. The grid wires are cylindrically symmetrical and thus do not intercept the electron stream preferentially. However, the axially extending grid supports located at multiple locations around the grid intercept the electron stream asymmetrically and cause stability problems as described below.
The fact that the grid posts intercept the electron stream asymmetrically would not be a problem if the electron stream could always be maintained in the same location across the grid volume. However, minute and, therefore, unavoidable changes in electrode geometry and in location of electron origins on the cathode can change the stream of electron trajectories significantly. Thus, when a given gauge is calibrated with the then existing electron trajectories, it remains accurate only so long as the electron trajectories through the grid volume do not change significantly. If the trajectories shift for any reason, then a fraction of the stream of electrons may impinge on a grid support causing a decrease in sensitivity or conversely, electrons trajectories which previously had been impinging on a grid support may shift and avoid the grid support, resulting in an increase in path length and, therefore, an increase in sensitivity. These shifts in electron trajectories cause inaccuracies in gauge indication with the root cause being the asymmetrically placed grid supports. Although these problems occur in the full-sized STABIL-ION Gauge design to a slight extent, the undesired effects become larger as the anode diameter is decreased. Thus, merely miniaturizing prior art geometries does not produce acceptable results.
If the STABIL-ION electrode geometry is decreased in size much below the standard intended dimensions listed in U.S. Pat. No. 5,128,617 another undesired effect is observed. The STABIL-ION design requires that the electron stream initially be aimed at an imaginary axis which is parallel to the axial ion collector but displaced radially from the ion collector as shown in FIG. 1 of U.S. Pat. No. 5,128,617.
Thus, when a small anode diameter is used, it is not possible to achieve long path lengths because the electron stream does not remain concentrated in a beam but is spread out as soon as the beam nears the extreme curvature of the small anode. Thus, it is very difficult to keep the electron stream from intersecting the grid posts in a miniature STABIL-ION design.
Prior art miniature ionization gauges are not able to measure very low pressures because of the so-called X-ray effect. Soft X-rays generated by electron impact on the grid cause electron ejection at the ion collector. This X-ray caused current is not pressure dependent and thus sets a lower limit on the pressure dependent ion collector current which is measured. Because the gauge sensitivity is reduced because of the small geometry, the X-ray effect is increased and the lowest measurable pressure is increased in prior art miniature ionization gauges.
It is well-known that the ion collector electrode exerts a repelling force on electrons depending on the distance of approach to the ion collector electrode. Thus, any slight shift in the trajectory of an electron relative to the ion collector electrode grows rapidly with time as the electron oscillates back and forth through the grid volume. In BA type gauges with a single ion collector this effect causes changes in how the electron stream interacts with the anode supports, thus leading to non-stable behavior. However, in accordance with the present invention, the repelling effects of the ion collector on electron trajectories inside the ion collection volume can be turned to advantage by utilizing the ion collector to repel electrons from the vicinity of an anode support.
Applicant has discovered that the problems described above can be avoided and that the total path length of electrons inside the anode volume can be greatly increased by locating an ion collector electrode parallel to and closely adjacent to each anode support posts. The multiple ion collector electrodes effectively repel electrons approaching the anode support posts and thus prevent premature collection of electrons on the posts. Thus, the electron path length is significantly increased in the present invention compared to that in prior art gauges of the same size. Increasing the path length of electrons inside the anode volume is highly desirable because increasing this portion of the path length increases the rate of ions created and, therefore, increases the gauge sensitivity proportionately.
In U.S. Pat. No. 3,353,048 the use of multiple ion collector electrodes is disclosed. The conventional single ion collector electrode, typically located on the axis of the grid, has been moved off center and duplicated for symmetry in this prior art device to provide space for a beam of molecules along the axis of the grid. However, the ion collector electrodes in U.S. Pat. No. 3,353,048 are not located adjacent to the anode support posts as in the present invention and, therefore, do not perform the essential functions required in the present invention of preventing premature electron collection.
In the article entitled "Modulated Bayard-Alpert Gauge", P. A. Redhead, Rev. of Sci. Inst., 1960, pp. 343-344, there is described a modulator gauge, this gauge being a Bayard-Alpert type gauge with a second electrode located in the grid volume. One of these electrodes is a conventional ion collector electrode disposed along the central axis of the grid volume and typically biased at ground potential. The other electrode is a so-called modulator electrode consisting of a small diameter wire located parallel to the ion collector electrode. In use, the potential of the modulator electrode is switched from grid to ground potential. When the modulator electrode is at grid potential, there is zero ion current to the modulator. When the modulator is at ground potential, a relatively large fraction of the ions within the grid volume are attracted to the modulator electrode, thus decreasing the ion collector current by 30 to 40%. The residual current to the ion collector electrode remains essentially constant as the modulator potential is switched. Thus, measurements with the modulator permit the residual current to be calculated separately from the true ion current. This gauge is referenced simply because of the second electrode being within the grid volume, the gauge, in fact, being unrelated to the present invention.